Universal mobile telecommunications system (UMTS) is a 3rd generation (3G) asynchronous mobile communication system operating in wideband code division multiple access (WCDMA) based on European systems, global system for mobile communications (GSM) and general packet radio services (GPRS). A long-term evolution (LTE) of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardized UMTS.
The 3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3GPP LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement.
Recently, there has been a surge of interest in supporting direct device-to-device (D2D) communication. This new interest is motivated by several factors, including the popularity of proximity-based services, driven largely by social networking applications, and the crushing data demands on cellular spectrum, much of which is localized traffic, and the under-utilization of uplink frequency bands. 3GPP is targeting the availability of D2D communication in LTE rel-12 to enable LTE become a competitive broadband communication technology for public safety networks, used by first responders. Due to the legacy issues and budget constraints, current public safety networks are still mainly based on obsolete 2G technologies while commercial networks are rapidly migrating to LTE. This evolution gap and the desire for enhanced services have led to global attempts to upgrade existing public safety networks. Compared to commercial networks, public safety networks have much more stringent service requirements (e.g., reliability and security) and also require direct communication, especially when cellular coverage fails or is not available. This essential direct mode feature is currently missing in LTE.
From a technical perspective, exploiting the nature proximity of communicating devices may provide multiple performance benefits. First, D2D user equipments (UEs) may enjoy high data rate and low end-to-end delay due to the short-range direct communication. Second, it is more resource-efficient for proximate UEs to communicate directly with each other, versus routing through an evolved NodeB (eNB) and possibly the core network. In particular, compared to normal downlink/uplink cellular communication, direct communication saves energy and improves radio resource utilization. Third, switching from an infrastructure path to a direct path offloads cellular traffic, alleviating congestion, and thus benefitting other non-D2D UEs as well. Other benefits may be envisioned such as range extension via UE-to-UE relaying.
While the UE performs a D2D operation with other UEs, handover of the UE may occur. In this case, information on radio resources used for a D2D operation should be informed from a source cell to a target cell. Accordingly, a method for handling radio resources used for a D2D operation upon mobility may be required.